In the application of optical imaging, one can fabricate a lens with variable focal distance by filling transparent liquid in a fluid chamber where at least one surface in the optical path contains a flexible membrane. When an extra amount of fluid is injected into the liquid chamber, the shape of the membrane changes with the increasing pressure and as a result, parallel light rays passing the membrane are bent and approximately meet at a tight position called focal point. When one varies the amount of fluid in the fluid chamber to change the shape of the flexible membrane, the focal point changes. Without changing the position of the fluidic lens from a CMOS or CCD image sensor, a fluidic lens can form images of objects at different distances by adjusting its focal distance. To achieve reliable and reproducible performance, the profile of the membrane should be determined by the fluidic pressure inside the lens chamber. Under a given pressure difference between the lens chamber and the ambient, the membrane should always produce a corresponding profile and therefore a focal distance. This requires that the membrane has to be highly elastic, absent of plastic deformation over the entire range of operation. Elastomer such as rubber and silicone can meet the above requirement, and a particular type of elastomer, poly(dimethylsiloxane) or PDMS, is attractive to fluidic adaptive lens because PDMS has high optical transparency.
Fluidic lenses can be incorporated into a myriad of optical systems that employ lenses such as cameras, microscopes, video monitors, video recorders, optical recording mechanisms, surveillance equipment, inspection equipment, agile imaging equipment, target tracking equipment, copy machines, scanners, etc. It would be advantageous if the fluidic lens or lens system could be implemented in zoom lens systems in a manner that reduced the need for complicated mechanical systems for controlling relative positioning of multiple lenses within the zoom lens systems. It also would be advantageous if a zoom lens system employing the fluidic lens or lens system could be compactly implemented on one or more types of physically small “electronic gadgets” such as cell phones, personal digital assistants (PDAs), or notebook computers.
Furthermore, because fluidic lens changes its focal distance in a similar manner as human eyes change their focus, fluidic lenses can be implanted into eyes to restore or enhance human vision. For implanted fluidic lens, it becomes cumbersome to use an external device such as a micro-pump or a micro actuator to inject or remove fluid into or out of the lens chamber. A more attractive lens structure is to form a flexible-shape fluid vessel without liquid inlet or outlet. In addition to the elastomer membrane in the direct optical path, one can incorporate additional elastic structures so that when an external force is employed, the overall shape of the vessel changes to a new equilibrium shape, and so does the focal distance of the fluidic lens. Compared with fluidic adaptive lenses using external actuators to change the fluidic volume inside the lens chamber, the design of variable shape liquid vessel tend to produce a smaller tuning range in focal distance. However, since the lens in human eye has a tuning range of no more than 10 dioptres, the design of variable shape fluidic vessel becomes attractive.
However, many elastomer materials used to form fluidic lenses do not produce a strong enough diffusion barrier for the fluid inside the fluidic vessel or lens chamber. When the chamber is pressured for an extended period (e.g. several hours or days) or heated to a higher than room temperature (e.g. 60 C), the fluid can permeate through the elastomer membrane. In other cases, the lens fluid can be incorporated into the matrix of the elastomer and cause wrinkling of the membrane. This effect is often called ‘swelling’. The molecular structure that makes the material highly elastic often shows strong tendency for fluid permeation. For instance, PDMS (e.g. Sylgard 184 from Dow Corning or Gelest 1.41 from Gelest, Inc.) has shown superb elastic properties and optical properties from UV, visible, and near infrared light, but the material is highly permeable. Under a positive chamber pressure of a few psi, fluid can diffuse through the PDMS membrane easily and form fluid droplets or patches of fluid on the outer surface of the membrane within hours. The problem of fluid permeation becomes a limiting factor for fluidic lenses.
Generally speaking, all polymers interact to a degree with fluids (liquids, gases), physico-chemically (i.e. swelling) and/or chemically. The extent of swelling depends primarily on the chemical similarity of the fluid and the polymer—“like swells like”. For instance, the all-hydrocarbon elastomer EPDM is swollen significantly by aliphatic hydrocarbon liquids, but negatively by water. The oil resistance of nitrile elastomer increases with acrylonitrile content of the base polymer, as does the glass transition temperature. For PDMS, a biocompatible elastomer particularly suitable for fluidics lenses and drug delivery devices, serious swelling occurs in hydrocarbon such as mineral oil and high permeation rate occurs with silicone oil, which is a popular lens fluid. For another class of lens fluid: polyphenyl ether (PPE) and polyphenyl thiol ether, swelling is not observed but permeation through the PDMS membrane still occurs. It is therefore advantageous to reduce or eliminate permeation through the lens membrane.